1. Field of the Invention
The present invention relates to a fuel supply device for direct methanol fuel cells. More particularly, the present invention relates to a fuel supply device for direct methanol fuel cells including a fuel tank having a fuel tank body that is capable of contracting and expanding, by which liquid fuel may be actively supplied in a predetermined amount to a fuel cell.
2. Description of the Related Art
Recently, as the use of notebook computers, mobile phones, PDAs, etc. increases, concern for energy sources of such portable electronic devices also increases. Energy sources of portable electronic devices should be small and should be able to continuously supply electric power even when such devices are carried for a long period of time. Conventional secondary cells, however, when recharged a single time, have a short usable period, and are further limited by a large size. In addition, conventional secondary cells are heavy, expensive, and cause pollution upon disposal. Fuel cells, in contrast, have an energy density about 3 times higher than that of secondary cells and thus, can be used for a long period of time by a single recharge. Also, fuel cells can provide a lightweight, small-sized energy source, and can be used semi-permanently by the refueling of a storing vessel. Further, fuel cells are environmentally friendly energy sources that do not cause pollution upon disposal. Therefore, fuel cells may make good energy sources for portable electronic devices.
Fuel cells are classified by electrolytes contained in the cells, including, for example, phosphoric acid fuel cells (PAFC) using phosphoric acid as an electrolyte, alkaline fuel cells using potassium hydroxide as an electrolyte, polymer electrolyte fuel cells (Proton Exchange Membrane Fuel Cell, PEMFC) using Nafion® Dow polymer as an electrolyte, molten carbonate fuel cells (MCFC) using lithium carbonate or potassium carbonate as an electrolyte, solid oxide fuel cells (SOFC) using an yttria-stabilized zirconia as an electrolyte, and direct methanol fuel cells (DMFC) using a polymer membrane as an electrolyte.
Among these, direct methanol fuel cells can directly use methanol as a fuel. Therefore, direct methanol fuel cells can be miniaturized and readily supplied with a fuel. Also, since direct methanol fuel cells have a high-density fuel output, research is under way for implementing direct methanol fuel cells as fuel cells for portable electronic devices.
As shown in FIG. 1, direct methanol fuel cells include a membrane electrode assembly, including an anode 2, a membrane 1 and a cathode 3. In the anode 2, methanol reacts with water to produce hydrogen ions and electrons. The hydrogen ions produced in the anode 2 are transferred to the cathode 3 through the electrolyte membrane 1. In the cathode 3, the hydrogen ions and electrons bind to oxygen to produce water. Such reactions are shown in the following Reaction Scheme (I).                Reaction Scheme I        Anode: CH3OH+H2O=CO2+6H++6e−        Cathode: 1.5O26H++6e−=3H2O        Total: CH3OH+1.5O2=CO2+3H2O        
The energy generated by the chemical reaction in the fuel cell, as shown above in Reaction Scheme (I), is supplied to an electronic device as electric energy.
Since the membrane electrode assembly of the fuel cell is affected by distribution of liquid fuel, infiltration of by-products, and control of liquid flow rate, the fuel delivery strongly influences the actions of the direct methanol fuel cell in portable electronic equipment. Correspondingly, the fuel delivery, or supply, device plays a very important role in the fuel cell.
A fuel supply device transports fuel (a mixture of methanol and water) from a fuel tank to the surface of the membrane electrode assembly and removes carbon dioxide and other by-products of the used fuel on the surface of the membrane electrode assembly. The fuel supply device for direct methanol fuel cells is generally required to uniformly supply fuel, have excellent reliability, be able to readily adjust flow rate, have low power consumption and be small-sized.
Fuel supply systems known in the prior art employ a small-sized pump to transfer fuel stored in a fuel tank through a micro-channel that is in contact with an anode of a membrane electrode assembly. These fuel supply systems sufficiently supply fuel and readily control the flow rate. However, since the systems include a pump as a main component, they are limited in ability to achieve a small size. Therefore, they are difficult to utilize in small-sized direct methanol fuel cells for small electronic devices such as mobile phones or PDAs. In addition, as is known in the art, with a same Reynolds number, the flow resistance of a small or micro-sized path is greater than that of a normal or broad path. Since greater electric power is required to transfer liquid fuel through the surface of a membrane electrode assembly having a small-sized path, the fuel supply systems described above have poor efficiency for consumed electric power.
Conventional active fuel supply systems for direct methanol fuel cells are further limited by complicated manufacturing processes, which increase expenses, since the pump mechanism for the fuel supply includes a number of components. In addition, it is difficult to recharge conventional active fuel supply systems for direct methanol fuel cells without contamination when recharging a fuel tank with fuel from a fuel supply source.
In order to reduce power consumption caused by an eddy current, it has been suggested to apply a passive supply system in a direct methanol fuel cell, such as an anode assembly directly contacting a mixture of methanol and water. Such a passive fuel supply system provides low power consumption, but a flow rate of supplied fuel is hard to control in such a system.
Therefore, uncontrollable operation of a direct methanol fuel cell employing a passive fuel supply system affects electronic devices in which they are used to some extent.
Also, passive fuel supply systems have a very slow fuel flow rate and a very small power output. Accordingly, passive fuel supply systems can only address needs of battery exchange apparatuses having low power consumption.